Protein S is a cofactor for the anticoagulant effects of activated protein C. We have discovered that individuals who lack functional protein S activity are prone to thromboembolic disease, presumably due to an inability to regulate blood clotting. However, while these individuals lack functional protein S activity, protein S antigenic levels are normal or mildly reduced. All or most of their protein S is complexed with C4-binding protein, an inhibitor of the complement system, and is not functionally active. In normals only 60% of the total protein S is complexed to C4-binding protein. We wish to determine why this shift in protein S distribution occurs in the familial functional protein S deficiency. To accomplish this goal, we will investigate the possible causes for the shift in the following order: First, initial determinations indicate that C4-binding protein is elevated in the functionally deficient individuals and we will determine if the levels of C4-binding protein in the patients' plasma are sufficient to explain the shift by mass action. This will require well-validated assays for C4-binding protein levels in plasma. Secondly, we find that activation of complement in vitro results in a shift from free protein S to bound. We will determine if complement activation is also involved in the in vivo changes observed in the patients and if the observed shift is mediated through binding of C4b to C4-binding protein. If complement activation does not prove responsible for the changes in protein S states observed in patients, we will isolate patient protein S and C4-binding and characterize these proteins both kinetically and structurally to determine if an abnormality in one of these proteins is responsible for the altered protein S distribution. We find that an acquired functional deficiency occurs in two major medical conditions which have thromboembolic complications - systemic lupus erythematosis and the nephrotic syndrome. Again, the protein S is shifted to the bound and inactive form. We will determine the mechanism(s) by which this shift occurs. We will investigate other medical conditions known to have thromboembolic complications to determine if acquired protein S deficiency occurs. These patient studies are the first step in the pursuit of our long-term goal which is the determination of whether or not monitoring protein S status may serve as a rational way of deciding which patients will benefit most from phrophylactic measures to prevent thromboembolic complications.